1. Field of the Invention
The present invention relates to a modulator for directly modulating a millimeter wave and a transmitter including the modulator. In this context, the millimeter wave includes a microwave.
2. Description of the Related Art
Several kinds of millimeter wave modulations have been known. In an up-converting modulation, a carrier is modulated at a low (intermediate) frequency and then the carrier is mixed or up-converted to a high output (transmission) frequency. This method is attended with a problem that the bandwidth is narrowed to attain high fidelity. For example, when the modulation is achieved with an intermediate frequency of 2 GHz to convert the carrier into a transmission frequency of 40 GHz, at most a modulation frequency bandwidth of about 100 MHz is obtained.
In a direct modulation (technique), a high-speed switching element, e.g., a PIN diode is arranged in a waveguide and the switching element is turned on or off by a signal to change impedance of a millimeter wave propagating the waveguide. When the PIN diode is switched at a high frequency, there appear L and C components, which equivalently forms an LC parallel resonance circuit. This resultantly prevents the high-speed switching and the bandwidth is limited to, for example, at most 100 MHz.